Ethanol for fuel has become a significant part of the transportation fuel mix. This is due to the rapidly increasing price of fossil fuel and as the desire to reduce the environmental impact of non-renewable fuels.
Ethanol is a simple molecule, non-toxic and miscible with gasoline. When ethanol is made by the fermentation of plant derived starches and sugars, it is considered to have a lower environmental impact than fossil fuels.
Ethanol is usually produced from starch or sugar by fermentation. In North America the feedstock is primarily corn, while in Brazil sugar cane is used. There are disadvantages to using potential food or feed plants to produce ethanol and the availability of such feedstock is limited by the overall available area of suitable agricultural land.
Many alternate feedstocks for ethanol production have been proposed. Among them is lignocellulosic biomass. This term includes cellulose containing agricultural and wood residues, purpose grown non-food crops, and a wide variety of biodegradable wastes.
Agricultural and wood residues and non-food crops have several economic and environmental advantages over corn and starch. Furthermore, some alternative crops such as Miscanthus, Switchgrass and hybrid Poplar can even grow on poor quality land not suitable for corn. Wood and agricultural residues have relatively low market value and have the potential to be high volume feedstocks for ethanol production.
Lignocellulosic biomass is composed of three major polymers: cellulose, hemicellulose and lignin. Cellulose makes up 40% to 60% of lignocellulosic biomass and is the desired target for ethanol production. Cellulose resembles starch in many ways. It is a homogeneous polymer made of linked glucose monomers, as is starch. Cellulose, however, is much more difficult to depolymerize than starch. This is due to a difference in the nature of the glucose linkages as well as the presence of hemicellulose and lignin. As a result, more severe conditions are needed to hydrolyze cellulose to glucose than are needed to hydrolyze starch.
One process for converting lignocellulosics to ethanol can be called the enzymatic hydrolysis process. This process requires five major unit operations: feed preparation, pretreatment, enzymatic hydrolysis, fermentation and distillation. Lignocellulosic biomass is chopped, cleaned, and ground to the desired size.
Pretreatment of the biomass opens up its structure, exposing the cellulose to the hydrolytic action of enzymes in the hydrolysis step. Pretreatment also increases the concentration of cellulose in the prehydrolysate, which improves the digestibility of the cellulose by enzymes.
In the enzymatic hydrolysis step, the prehydrolysate obtained in the pretreatment step is cooled to about 40° C. to 60° C., cellulase enzymes are added and the hydrolysis is allowed to continue to achieve the desired conversion of cellulose to glucose. Fermentation of the sugars in the hydrolysate by yeast is the next step.
In the final step, ethanol is recovered by distillation of the fermented mash and dehydration of ethanol to the desired concentration. Many different configurations for this step are practiced in the industry.
Lignin is a potent inhibitor of hydrolysis and some soluble lignin derivatives inhibit the fermentation process. Thus, it is desirable to use a lignocellulosic feedstock which is low in lignin. The lignin content of corncobs, (less than 8% by weight) is low, which would make this a good biomass feedstock for the production of ethanol. However the hemicellulose content of corncobs is high, almost 30% of the total dry matter. Moreover, much of the hemicellulose is acetylated. The dissolution of hemicellulose leads to the formation of acetic acid, a powerful inhibitor of the yeast fermentation process used to produce ethanol. This is a problem, since the acid remains in the pretreated biomass and carries through to the hydrolysis and fermentation steps.
In known pretreatment processes, mineral acids such as sulfuric acid, are added to the biomass for hydrolysis of the biomass components. This approach is disadvantageous since large amounts of water are required to flush the acid from the pretreated cellulose prior to the enzymatic hydrolysis step. Moreover, acid pretreatment also leads to the dissolution of hemicellulose and the release of acetic acid, as discussed above. Many of the compounds released in the pretreatment step, such as acetic acid, hemicellulose and many hemicellulose degradation products are inhibitors of and retard the downstream fermentation process. This results in increased capital equipment costs and frequently incomplete conversion of the glucose to ethanol. Therefore, it would be desirable to reduce the hemicellulose content of the feedstock for the enzymatic hydrolysis step to aid in cellulose digestion.
In an alternate approach a steam gun cellulose pretreatment is used. Biomass ground to the desired size is subjected to steam under pressure and at elevated temperatures. The pressure is then released rapidly by way of a fast acting valve, leading to an explosion of the cooked biomass material. In this approach the steam explosion may be a batch process or a continuous process.
Hemicellulosic hydrolysate composition is highly dependent on feedstock type and pretreatment methodology. Significant compositional differences are observed among hemicellulosic biomasses. The hemicellulose polysaccharides are quite heterogeneous because they can contain hexoses, pentoses and organic acids e.g. acetic acid (1-3). Softwood xylan is weakly acetylated while hardwood xylan is highly acetylated (4). Acetic acid levels in hemicellulose hydrolysates of herbaceous species are approximately 50% lower than typically observed in similarly prepared hardwood hydrolysates because of the lower degree of hemicellulose acetylation in herbaceous species relative to hardwood (5, 6). Average values of acetyl content recorded in Table 1 show that some of the highest levels of hemicellulose acetylation are found in corn residues such as cornstalks and corn cobs (7-8).
TABLE 1Acetyl content of lignocellulosic biomassesSourcesAcetyl (%, average)Softwoods1.4Hardwoods3.7Grasses1.9Wheat straw2.9Corncobs3.1Cornstalks4.6Extractive-Free Dry Basis Analysis
Pretreatment of highly acetylated lignocellulosic materials such as corn cobs can be performed in absence of mineral acids i.e. autohydrolysis (9-11). Our results showed that high temperature steam explosion pretreatment of corn cobs produces prehydrolysates with a pH that ranges from pH 3.5 to pH 3.8. Results plotted on FIG. 1 showed that, under similar pretreatment conditions, a dosage of 1.6% to 1.8% (w/w) of sulphuric acid is required to produce Miscanthus (grasses) prehydrolysate with the same pH values as corncob prehydrolysate.
High temperature steam pretreatment converts acetyl groups in hemicellulose molecules into acetic acid which then hydrolyses xylan polymer into xylose oligomers and monomers (12, 13). The formation rate and concentration of these compounds in the resulting prehydrolysates depend on the autohydrolysis conditions e.g. temperature and reaction time which can be controlled to produce the desired level of acetic acid and hemicellulose hydrolysis (12-15).
The acetic acid content of 20% consistency corn cobs hydrolysates produced under optimum pretreatment conditions ranges from 0.6% to 1.2% (w/v) after enzymatic hydrolysis. The content of acetic acid has to be controlled since it is considered as one of the dominant fermentation inhibitors in hydrolysates of hemicellulosic biomasses (16-23). The presence of acetic acid significantly reduces the performance of ethanologenic organisms. It inhibits fermentation in an exponential way (24). The mechanism of its toxicity involves the acidification of the cytoplasm and modification of certain enzymes of glycolysis (25). Steam explosion pretreatment trials carried out with other hemicellulose biomasses such as Miscanthus showed that the selected range of pretreatment severities is specific to corncobs (FIG. 2)
Fermentation of 20% consistency corncobs hydrolysates produced under optimum conditions is shown in FIGS. 4A and 4B. 54 g/l of ethanol were produced in 25 h to 30 h at pH values that ranged from pH 5.3 to pH 5.9. This production of ethanol corresponded to 94% of the maximum theoretical glucose to ethanol conversion.
In the presence of 0.6% acetic acid, this ethanol production was reached in 25 h following an initial pH adjustment at pH values that ranged from pH 5.3 to pH 5.9, using 30% ammonium hydroxide (FIG. 4A). In the presence of 1.2% acetic acid, similar ethanol production was reached in 30 h following an initial pH adjustment at pH 5.6 (FIG. 4B).
These results are in accordance with results reported in the literature which showed that the tolerance to moderate acetic acid concentrations of the yeast Saccharomyces cerevisiae is strongly pH dependant. Carrying out fermentation of a complex medium at pH value higher than the pka of acetic acid (pKa 4.75) substantially reduces inhibition by acetic acid (21-27).
Foody in U.S. Pat. No. 4,461,648 describes a method of increasing the accessibility of cellulose in wood by steam explosion. However the authors state that a temperature of 205° C. for 15 minutes is required to achieve the maximum yield of glucose. The yield of glucose has a maximum of only 33%. An acid catalyst is needed.
U.S. Pat. No. 6,090,595 (Foody) teaches that the key to processing corncobs and other agricultural residues is to select the correct feedstock. No method of modifying the processing conditions to improve glucose yield is described. Pretreatment is carried out at pH 0.5 to 2.5.
Foody et al. in patent application US20090023187 focus on the separation of sugars by Simulated Moving Bed chromatography. Pretreatment is described as taking place at low pH.
Wingerson in U.S. Pat. No. 6,419,788 describes the pretreatment of biomass with oxygen at a pH of 8 to 13.
In each of these patents the conditions of pretreatment require extended treatment time or the use of chemicals to achieve acceptable recovery of glucose from lignocellulosic biomass.